Air Quality Guidelines - World Health Organization Regional Office ...

effects of airborne nitrogen pollutants on vegetation
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The empirical approach has been used to establish guidelines for excess
nitrogen deposition on natural and seminatural vegetation. It was decided
not to include the results of the mass balance approach with nitrogen as a
nutrient for non-forest ecosystems, because essential data are missing. The
acidifying effects of airborne nitrogen are incorporated in the guidelines for
excess acidity based on steady state mass balance models (see Chapter 13).
EVALUATION OF CRITICAL LOADS
The main aim of this evaluation was to update the guideline for airborne
nitrogen deposition on vegetation, which was estimated at 30 kg/ha per
year for sensitive vegetation (2). Since 1987, significant progress has been
made in understanding the ecological effects of nitrogen deposition on
several types of vegetation. Critical loads of nitrogen have been formulated
on an empirical basis by observing changes in the vegetation, fauna and
biodiversity (3, 4). Experiments under controlled and field conditions, and
comparisons of vegetation and fauna composition in time and space, are
used to detect changes in ecosystem structure (5–7).
Changes in plant development and in species composition or dominance
have been used as a “detectable change” for the impacts of excess nitrogen
deposition, but in some cases a change in ecosystem function, such as
nitrogen leaching or nitrogen accumulation, has been used. The results of
dynamic ecosystem models, integrating both biotic and abiotic processes,
are also used where available. Based on these data, guidelines for nitrogen
deposition (critical loads) have been presented for receptor groups of natural
and seminatural ecosystems, namely:
• wetlands, bogs and softwater lakes
• species-rich grasslands
• heathlands
• forest ecosystems (including tree health and biodiversity).
Critical loads have been defined within a range per ecosystem, because
of (a) real intra-ecosystem variation within and between countries, (b) the
range of experimental treatment where an effect was observed or not observed,
or (c) uncertainties in deposition values, where critical loads are
based on field observations. The reliability of the figures presented is shown
in Table 36.
It is advised, where insufficient national data are available, to use the lower,
middle or upper part of the ranges of the nitrogen critical loads for terrestrial